Ripgrep spawn helper rewritten with streaming truncated buffers and richer failure handling

The internal component that runs ripgrep (used to power search/grep) was rewritten. Output from the search process is now collected with a streaming buffer instead of simple string concatenation, there is a new error path for when writing to the process's input fails, and failures are now reported with distinct error types depending on the cause: file-not-found, an internal range error, or a generic spawn failure.